Polyimide gas separation membranes

ABSTRACT

A highly permeable aromatic polyimide gas separation membrane and the process of using it are disclosed. The aromatic polyimide has the formula ##STR1## wherein --Ar-- is ##STR2## or mixtures thereof, --X, --X 1 , --X 2  and --X 3  independently are alkyl groups having 1 to 6 carbon atoms or aromatic groups having 6 to 13 carbon atoms, and --Z is --H, --X, --X 1 , --X 2 , or --X 3  ; and R is ##STR3## or mixtures thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to certain substituted aromatic polyimidegas separation membranes and the process of using them.

2. Prior Art

U.S. Pat. No. 3,822,202 and U.S. Pat. No. 3,899,309 disclose aromaticpolyimide gas separation membranes in which the molecular structure issuch that the molecules in the polymer are unable to pack densely, and,therefore, have high gas permeation rates.

U.S. Pat. No. 4,113,628 and U.K. No. 1,435,151 disclose aromaticpolyimide gas separation membranes prepared from polyamide acidmembranes.

EPO No. 132,221 and EPO No. 141,781 disclose substituted aromaticpolyimides and photochemically crosslinked compositions thereof, but notfor use as gas separation membranes.

U.S. Pat. Nos. 4,370,290, 4,460,526, 4,474,662, 4,512,893, U.K. No.2,098,994, U.K. No. 2,101,137, and U.K No. 2,102,333 disclosemicroporous aromatic polyimide membranes and the process from which theyare prepared.

U.S. Pat. No. 4,486,376 and U.K. No. 2,104,832 disclose gas separationmembranes made from a microporous aromatic polyimide support treatedwith modifying agents.

U.S. Pat. Nos. 4,378,324, 4,440,643, 4,474,858, 4,485,056, 4,528,004,U.K No. 2,102,726, U.K. No. 2,104,411, and U.K. No. 2,104,905 disclosegas separation membranes made from a microporous aromatic polyimidesupport coated with an aromatic polyamide acid or aromatic polyimide.

U.S. Pat. No. 4,378,400 and EPO No. 43,265 disclose aromatic polyimidegas separation membranes in which the molecular structure is such thatthe molecules in the polymer can pack densely.

EPO No. 113,574 discloses gas separation membranes made from aromaticpolyetherimides.

EPO No. 143,552 discloses gas separation membranes made from amicroporous aromatic polyimide support coated with a crosslinkedsilicone resin film.

EPO No. 125,908 discloses aromatic polyimide reverse osmosis membranes.

SUMMARY OF THE INVENTION

The present invention relates to aromatic polyimide gas separationmembranes in which the diamines are rigid and are substituted onessentially all of the positions ortho to the amino substituents and theacid anhydride groups are essentially all attached to rigid aromaticmoieties. The membranes exhibit very high permeation to gases whilestill being able to effectively separate several combinations of gases.

DETAILED DESCRIPTION

The present invention relates to the discovery that aromatic polyimidesprepared by polycondensation of dianhydrides with phenylene diamineshaving substituents on all positions ortho to the amine functions formmembranes with exceptional gas permeability. The gas permeabilityincreases substantially if structurally rigid dianhydrides are used incombination with the substituted diamines. These increases inproductivity of these membranes is believed to be due to the highmolecular free volume in the polymer structure resulting from the rigidnature of the rotationally hindered polymer chains.

Generally, extremely high gas permeation through dense polymericmembranes is found only with low glass transition temperature (Tg)materials, such as silicone rubbers and many polyolefin polymers. Thelow Tg materials are generally only useful as the dense separating layerin composite membranes, where a supporting porous membrane substructureprovides structural integrity.

High Tg polymeric materials, found in the prior art, do not possessextremely high gas permeabilities. Examples reported involve mostlyunsubstituted, or partially substituted, polymers subject to formingclose chain packing during fabrication and/or subsequent operation.

The present invention circumvents the above shortcomings and providesexceptionally high permeability dense membranes using high Tg aromaticpolyimides containing the repeating unit: ##STR4## where

--Ar-- is ##STR5## or mixture thereof,

R is ##STR6## or mixture thereof

--X, --X₁, --X₂, and --X₃ are independently alkyl groups having 1 to 6carbon atoms preferably methyl or ethyl, or aromatic groups of 6 to 13carbon atoms, and

--Z is --H, --X, --X₁, --X₂, or --X₃.

The multiple substitutions ortho to the amines in the above illustratedstructures sterically restricts free rotation around the imide linkage.In addition, the dianhydrides used in this invention are structurallyrigid. This causes the chains in the polyimides of this invention to beextremely rigid. The greatly enhanced permeability observed in thesemembranes may be attributed to the rigid nature of the polyimide. Theability of these membranes to effect useful gas separations fromulticomponent mixtures may be due to the highly rigid nature of thesepolyimides allowing the optimum molecular free volume in the polymer.

The gas permeabilities of non-rigid polymers, such as polyethersulfones, polyphenylene oxides, polyamides and polyimides, reportedbefore are generally ordered: H₂ >He>CO₂ >O₂ >CH₄ ≧N₂. The rigidpolyimides reported in this patent have the gas permeabilities order:CO₂ >He≧H₂ >O₂ >N₂ ≧CH₄. This is similar to the order found for otherultra high permeability polymers, such as the silicone rubbers.

As preferred examples, some of the fully cyclized polyimides of thisinvention are soluble in ordinary organic solvents. This is a greatadvantage for the ease of fabrication of industrially useful membranes.The polyimides reported in this invention range from soluble to solventresistant and can be fabricated into membranes using known techniques.For example, the soluble polyimides can be solution cast on poroussolvent resistant substrates to serve as the dense separating layer of acomposite membrane. The soluble examples can be solution cast as denseor asymmetric films. Insoluble examples can be cast into membranes fromtheir polyamic acid form, followed by chemical or thermal cyclization.

The polyimides described in this invention have high inherent thermalstabilities. They are stable to 400° C. in air or inert atmospheres. Theglass transition temperatures of these polyimides are generally above300° C. The high temperature characteristics of these polyimides canhelp to prevent the membrane compaction problems observed in otherpolymers to even moderate temperatures.

The membranes of the present invention are generally useful inseparating mixtures of gases. Specific examples of such separationsinclude recovery of hydrogen in oil refineries and in ammonia plants,separation of carbon monoxide and hydrogen in syngas systems, separationof carbon dioxide or hydrogen sulfide from hydrocarbons, and enrichmentof oxygen and nitrogen from air for increased combustion or inertingstreams, respectively.

EXAMPLES Example 1

Dimethylsulfoxide (500 ml) was added to4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) (6F, 88.87 g,0.20 mol) and 2,4,5-trimethyl-1,3-phenylene diamine (DAM, 30.01 g, 0.20mol) at 50° C. in an inert atmosphere with stirring. After stirring at50° C. for 2 hours, a mixture of acetic anhydride (75.8 ml, 0.8032 mol)and triethylamine (112.94 ml, 0.8104 mol) were added and the reactionallowed to stir for an additional 2 hours at 50° C. under an inertatmosphere. The viscous reaction solution was precipitated in methanol,and the white solid was filtered and dried in a vacuum oven overnight atroom temperature, at 110° C. for 2 hours, and at 220° C. for 5 hours.This gave 108.8 g of off-white polyimide. This polyimide is soluble atgreater than 20% solids (based on polymer weight) in acetone,dichloromethane, N-methyl pyrrolidone, m-cresol, and dimethyl acetamide.

Example 2

Films of the polyimide prepared in Example 1 were cast from a 10%polymer solution based on weight in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 80° C. with a 15-mil(38×10⁵ m) knife gap. The films were dried on the plate at roomtemperature, stripped off the plate and dried at room temperatureovernight in a vacuum oven, and then at 200° C. for 6 hours in a vacuumoven. The films were then tested for mixed gas CO₂ /methane (50/50 moleratio) permeabilities at 240 psig (16.5×10⁵ Pa), 25° C. The results arereported below:

CO₂ Productivity: 72,636 centiBarrer

CH₄ Productivity: 3,792 centiBarrer

CO₂ /CH₄ Selectivity: 19.1

and

CO₂ Productivity: 71,435 centiBarrer

CH₄ Productivity: 3,776 centiBarrer

CO₂ /CH₄ Selectivity: 18.9

Du Pont TEFLON® dry lubricant contains a fluorocarbon telomer whichreduces the adhesion of the membrane to the glass plate. ##EQU1##

Example 3

Films of the polyimide prepared in Example 1 were cast from 15% polymersolution (based on weight) in N-methyl pyrrolidone onto a glass platetreated with Du Pont TEFLON® dry lubricant at 100° C. with a 15 mil(38×10⁵ m) knife gap. The films were dried on the plate at 100° C. for15 minutes, cooled, stripped off the plate and dried at 100° C.overnight in a vacuum oven. The films were then tested for mixed gas CO₂/methane (50/50 mole ratio) permeabilities at 240 psig (16.5×10⁵ Pa),25° C. The results are reported below:

CO₂ Productivity: 40,888 centiBarrer

CH₄ Productivity: 2,013 centiBarrer

CO₂ /CH₄ Selectivity: 20.3

and

CO₂ Productivity: 38,583 centiBarrer

CH₄ Productivity: 2,167 centiBarrer

CO₂ /CH₄ Selectivity: 17.8

The films prepared above were extracted in MeOH:H₂ O (50:50) for 3 days,and twice in MeOH for 3 hours each. These films were dried in a vacuumoven overnight at room temperature, and at 100° C. in a vacuum oven for24 hours. The films were then tested for mixed gas CO₂ /methane (50/50mole ratio) permeabilities at 400 psig (27.6×10⁵ Pa), 35° C. The resultsare reported below:

CO₂ Productivity: 69,080 centiBarrer

CH₄ Productivity: 4,881 centiBarrer

CO₂ /CH₄ Selectivity: 14.2

The extracted films prepared above were tested for mixed gas CO₂/methane (50/50 mole ratio) permeabilities at 400 psig (27.6×10⁵ Pa),90° C. The results after 48 hours of testing are reported below:

CO₂ Productivity: 57,105 centiBarrer

CO₂ /CH₄ Selectivity: 8.0

The extracted films prepared above were tested for mixed gashydrogen/methane (50/50 mole ratio) permeabilities of 400 psig (27.6×10⁵Pa), 35° C. The results are reported below:

H₂ Productivity: 43,340 centiBarrer

CH₄ Selectivity: 3,570 centiBarrer

H₂ /CH₄ Selectivity: 12.1

Example 4

Films of the polyimide prepared in Example 1 were cast from a 15%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 80° C. with a 15 mil(3.8×10⁻⁵ m) knife gap. The films were dried on the plate at 80° C. for25 minutes, cooled to room temperature, stripped off the plate and,dried at room temperature overnight in a vacuum oven, and at 120° C. for4 hours in a vacuum oven. The films were then tested for mixed gas O₂/N₂ (21/79 mole ratio) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C.The results are reported below:

O₂ Productivity: 13,000 centiBarrer

O₂ /N₂ Selectivity: 3.0

The films prepared above were heat treated in a vacuum oven at 250° C.for 4 hours. The films were then tested for mixed gas O₂ /N₂ (21/79 moleratio) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C. The results arereported below:

O₂ Productivity: 9,300 centiBarrer

O₂ /N₂ Productivity: 3.2

Example 5

Films of the polyimide prepared in Example 1 were cast from a 10%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 100° C. with a10-mil (2.54×10⁻⁴ m) knife gap. A slanted glass dust cover was placedover the film and the film dried on the plate at 100° C. for 10 minutesand then at 80° C. in a vacuum oven overnight. The films were cooled toroom temperature, stripped off the plate and tested for mixed gas O₂ /N₂(21/79 mole ratio) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C. Theresults are reported below:

O₂ Productivity: 12,000 centiBarrer

O₂ /N₂ Selectivity: 3.6

Example 6

N,N-Dimethylacetamide (750 ml) was added to a mixture of 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA, 65.44 g, 0.30 mol) and2,4,6-trimethyl-1,3-phenylene diamine (DAM, 45.07 g, 0.30 mol) at 50° C.in an inert atmosphere with stirring. After stirring at 50° C. for 1hour, a solution of acetic anhydride (113.7 ml, 1.21 mol) andtriethylamine (169.41 ml, 1.22 mol) was added to the light goldenviscous solution with stirring. A bright yellow solid came out ofsolution and slowly redissolved to form a dark brown viscous solution.The reaction solution was allowed to stir at 50° C. in an inertatmosphere for 3 hours, and was then precipitated in methanol. The lightsolid was collected by filtration, dried overnight at room temperaturein a vacuum oven, and then dried at 200° C. in a vacuum oven for 5 hoursto give 98.5 g product. This polyimide is soluble at 10% solids (basedon weight) in N-methyl pyrrolidone.

Example 7

Films of the polyimide prepared in Example 6 were cast from a 10%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 80° C. with a 15 mil(3.8×10⁻⁵ m) knife gap. The films were dried on the plate at ambienttemperature, stripped off the plate, and dried at room temperature in avacuum oven overnight, and then at 200° C. for 6 hours in a vacuum oven.The films were then tested for mixed gas CO₂ /methane (50/50 mole ratio)permeabilities at 245 psig (17×10⁵ Pa), 25° C. The results for a film 1mil thick are reported below:

CO₂ Productivity: 62,285 centiBarrer

CH₄ Productivity: 3,418 centiBarrer

CO₂ /CH₄ Selectivity: 18.2

Example 8

Films of the polyimide prepared in Example 6 were cast from a 10%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 100° C. with a 15mil (3.8×10⁻⁵ m) knife gap. The films were dried on the plate at 100° C.for 15 minutes, cooled to room temperature, stripped off the plate anddried at 100° C. overnight in a vacuum oven. The films were then testedfor mixed gas CO₂ /methane (50/50 mole ratio) permeabilities at 235 psig(16.2×10⁵ Pa), 25° C. The results are reported below:

CO₂ Productivity: 13,141 centiBarrer

CH₄ Productivity: 631 centiBarrer

CO₂ /CH₄ Selectivity: 20.8

Example 9

Films of the polyimide prepared in Example 6 were cast from a 10%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 80° C. with a 15-mil (3.8×10⁻⁵ m) knife gap. The films were dried on the plate at 80° C.for 30 minutes, cooled to room temperature, stripped off the plate, anddried at room temperature overnight in a vacuum oven and at 120° C. for4 hours in a vacuum oven. The films were then tested for mixed gas O₂/N₂ (21/79 mole ratio) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C.The results are reported below:

O₂ Productivity: 4,300 centiBarrer

O₂ /N₂ Selectivity: 3.2

Example 10

Films of the polyimide prepared in Example 6 were cast from a 10%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 50° C. with a 12-mil(3.0×10⁻⁵ m) knife gap. A slanted glass dust cover was placed over thefilm and the film dried on the plate at 50° C. for 40 minutes, and thenat 80° C. in a vacuum oven overnight. The films were cooled to roomtemperature, stripped off the plate and tested for mixed gas O₂ /N₂(21/79) (mole ratio) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C.The results are reported below:

O₂ Productivity: 8,100 centiBarrer

O₂ /N₂ Selectivity: 3.6

Example 11

To a solution of 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA, 58.89g, 0.27 mol) and 4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride)(6F, 13.32 g, 0.03 mol) in N,N-dimethylacetamide (500 ml) was added withstirring at 50° C. in an inert atmosphere a solution of2,4,6-trimethyl-1,3-phenylene diamine (DAM, 45.07 g, 0.30 mol) inN,N-dimethylacetamide (250 ml). The resulting golden viscous solutionwas stirred at 50° C. for 2 hours and then a solution of aceticanhydride (113.7 ml, 1.21 mol) and triethylamine (169.41 ml, 1.215 mol)was added with stirring. A bright yellow solid formed and slowly wentback into solution to form a clear copper colored viscous liquid. Afterstirring for 2 hours at 50° C., the reaction solution was precipitatedin methanol, the resulting light solid collected by filtration and driedovernight at ambient temperature and then at 200° C. for 5 hours in avacuum oven to give 105.3 g product.

Example 12

Films of the polyimide prepared in Example 11 were cast from a 15%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate at 50° C. with a 15-mil (3.8×10⁻⁵ m) knife gap. After drying onthe plate at 50° C. for 10 minutes, the films were stripped off theplate and dried in a vacuum oven overnight at 80° C. The films weretested for mixed gas CO₂ /methane (50/50 mole ratio) permeabilities at150 psig (10.3×10⁵ Pa), 25° C. The results are reported below:

CO₂ Productivity: 8,869 centiBarrer

CO₂ /CH₄ Selectivity: 23

Films prepared as described above were then extracted overnight withrunning distilled water and dried in a vacuum oven at 80° C. The filmswere tested for mixed gas CO₂ /methane (50/50 mole ratio) permeabilitiesat 150 psig (10.3×10⁵ Pa), 25° C. The results are reported below:

CO₂ Productivity: 41,275 centiBarrer

CO₂ /CH₄ Selectivity: 20.5

Example 13

Films of the polyimide prepared in Example 11 were cast from a 15%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 100° C. with a 15mil (3.8×10⁻⁵ m) knife gap. The films were dried on the plate at 100° C.for 15 minutes, cooled to room temperature, stripped off the plate anddried at 100° C. overnight in a vacuum oven. The films were then testedfor mixed gas CO₂ /methane (50/50 mole ratio) permeabilities at 245 psig(16.9×10⁵ Pa), 25° C. The results are reported below:

CO₂ Productivity: 12,469 centiBarrer

CH₄ Productivity: 796 centiBarrer

CO₂ /CH₄ Selectivity: 15.6

Example 14

To a solution of 2,3,5,6-tetramethyl-1,4-phenylene diamine (16.43 g,0.10 mol) in N-methyl pyrrolidone (250 ml) was portionwise added4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) (6F, 44.4 g,0.10 mol, four portions, last portion washed in with 100 ml N-methylpyrrolidone) under an inert atmosphere with stirring at roomtemperature. After the golden solution had stirred for three hours atroom temperature, a solution of acetic anhydride (37.9 ml, 0.4016 mol)and triethylamine (56.47 ml, 0.4052 mol) was added with rapid stirring.The resultant orange solution was stirred at room temperature for twohours and then precipitated in methanol. The white solid was collectedby filtration, air dried, and then dried in a vacuum oven at roomtemperature overnight, at 100° C. for 3 hours, and at 250° C. for 3hours to give 54.2 g product.

Example 15

Films of the polyimide prepared in Example 14 were cast from a 15%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 75° C. with a 15-mil(38.4×10⁻⁵ m) knife gap. The films were dried on the plate at 75° C. for25 minutes, cooled to room temperature, stripped off the plate and driedat room temperature overnight, and at 120° C. for 4 hours, and at 220°C. for 3 hours in a vacuum oven. The films were then tested for mixedgas CO₂ /methane (50/50 mole ratio) permeabilities at 245 psig (16.9×10⁵Pa), 25° C. The results are reported below:

CO₂ Productivity: 38,058 centiBarrer

CH₄ Productivity: 1,537 centiBarrer

CO₂ /CH₄ Selectivity: 24.8

The films prepared above were tested for mixed gas O₂ /N₂ (21/79 moleratio) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C. The results arereported below:

O₂ Productivity: 7,300 CentiBarrer

O₂ /N₂ Selectivity: 3.2

The films prepared above were consecutively tested for pure gas helium,nitrogen, and carbon dioxide permeabilities at 400 psig (27.6×10⁵ Pa),25° C. The results are reported below:

He Productivity: 53,000 centiBarrer

N₂ Productivity: 2,300 centiBarrer

CO₂ Productivity: 76,000 centiBarrer

He/N₂ Selectivity: 23

CO₂ /N₂ Selectivity: 33

Example 16

To a solution of 2,3,5,6-tetramethyl-1,4-phenylene diamine (16.43 g,0.10 mol) and 2,4,6-trimethyl-1,3-phenylene diamine (15.02 g, 0.10 mol)in N-methyl pyrrolidone (300 ml) was portionwise added4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) (6F, 88.8 g,0.20 mol, added in 9 portions, last portion washed in with N-methylpyrrolidone (166 ml) under an inert atmosphere with stirring at roomtemperature. The clear yellow solution was stirred at room temperaturefor 3 hours, becoming so viscous that additional N-methyl pyrrolidone(200 ml) was added. A solution of acetic anhydride (75.8 ml, 0.8032 mol)and triethylamine (112.94 ml, 0.8104 mol) was added with rapid stirringat room temperature. A white solid came out of solution, but slowlyredissolved to form a viscous orange-yellow solution. After stirring for2 hours at room temperature, the reaction solution was precipitated inmethanol. The resulting off-white solid was collected, washed withadditional methanol, air dried and dried in a vacuum oven at roomtemperature overnight, at 100° C. for 3 hours, and at 250° C. for 3hours, to give 109.4 g product.

Example 17

Films of the polyimide prepared in Example 16 were cast from a 15%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 80° C. with a 15 mil(3.8×10⁻⁵ ) knife gap. The films were dried on a plate at 80° C. for 25minutes, cooled to room temperature, stripped off the plate and dried ina vacuum oven at room temperature overnight and at 120° C. for 4 hours.The films were then tested for mixed gas O₂ /N₂ (21/79 mole ratio)permeabilities at 300 psig (20.7×10⁵ Pa), 25° C. The results arereported below:

O₂ Productivity: 5,400 centiBarrer

O₂ /N₂ Selectivity: 3.6

The films prepared above were tested for mixed gas CO₂ /CH₄ (50:50 moleratio) permeabilities at 240 psig (16.5×10⁵ Pa), 25° C. The results arereported below:

CO₂ Productivity: 32,000 centiBarrer

CH₄ Productivity: 1,300 centiBarrer

CO₂ /CH₄ Selectivity: 24.4

Example 18

To a solution of 2,3,5,6-tetramethyl-1,4-phenylene diamine (8.21 g, 0.05mol) and 2,4,6-trimethyl-1,3-phenylene diamine (7.51 g, 0.05 mol) inN-methyl pyrrolidone (300 ml) was added 1,2,4,5-benzenetetracarboxylicdianhydride (10.91 g, 0.05 mol, washed in with N-methyl pyrrolidone (50ml)) under an inert atmosphere with stirring at room temperature. Afterthis yellow solution had stirred for 30 minutes,4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) (22.2 g, 0.05mol, added in 2 portions, last portion washed in with N-methylpyrrolidone (100 ml)) was portionwise added at room temperature and theresulting yellow solution stirred for 3 hours. A solution of aceticanhydride (37.9 ml, 0.40 mol) and triethylamine (56.47 ml, 0.41 mol) wasadded with rapid stirring at room temperature. A white solidprecipitated, but rapidly redissolved to form a gold solution. Afterstirring for 2 hours, the reaction solution was precipitated inmethanol. The resulting off-white solid was collected by filtration, airdried, and dried in a vacuum oven at room temperature overnight, at 100°C. for 3 hours, and at 250° C. for 3 hours to give 44.0 g product.

Example 19

Films of the polyimide prepared in Example 18 were cast from a 15%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 80° C. with a 15-mil(3.8×10⁻⁵ m) knife gap. The films were dried on a plate at 80° C. for 30minutes, cooled to room temperature, stripped off the plate and dried ina vacuum oven at room temperature overnight and at 120° C. for 5 hours.The films were then tested for mixed gas O₂ /N₂ (21/79 mole ratio)permeabilities at 300 psig (20.7×10⁵ Pa), 25° C. The results arereported below:

O₂ Productivity: 4,900 centiBarrer

O₂ /N₂ Selectivity: 3.5

The films prepared above were tested for mixed gas CO₂ /CH₄ (50:50 moleratio) permeabilities at 245 psig (16.9×10⁵ Pa), 25° C. The results arereported below:

CO₂ Productivity: 27,800 centiBarrer

CH₄ Productivity: 1,200 centiBarrer

CO₂ /CH₄ Selectivity: 23.2

Example 20

To a solution of 2,4,6-trimethyl-1,3-phenylene diamine (30.01 g, 0.20mol) in N-methyl pyrrolidone (500 ml) was added1,2,4,5-benzenetetracarboxylic dianhydride (21.81 g, 0.10 mol) under aninert atmosphere with stirring at 50° C. After stirring for 30 minutes,4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) (44.4 g, 0.10mol, added in 2 portions, last portion washed in with N-methylpyrrolidone (250 ml)) was portionwise added with stirring at 50° C. Theorange reaction solution was allowed to stir at 50° C. overnight. Asolution of acetic anhydride (75.84 ml, 0.80 mol) and triethylamine(112.94 ml, 0.81 mol) was added with rapid stirring causing the reactionsolution to turn brown. After stirring at 50° C. for 2 hours, thereaction solution was precipitated in methanol. The resulting off-whitesolid was collected by filtration, washed with methanol, air dried anddried in a vacuum oven at room temperature overnight, at 100° C. for 4hours, and at 250° C. for 3 hours to give 86.8 g product.

Example 21

Films of the polyimide prepared in Example 20 were cast from a 15%polymer solution (based on weight) in N-methyl pyrrolidone onto a glassplate treated with Du Pont TEFLON® dry lubricant at 80° C. with a 15 mil(3.8×10⁻⁵) knife gap. The films were dried on a plate at 80° C. for 25minutes, stripped off the plate and dried in a vacuum oven at roomtemperature overnight and at 120° C. for 4 hours. The films wereconsecutively tested for pure gas helium, nitrogen, and carbon dioxidepermeabilities at 400 psig (2.7×10⁵ Pa), 25° C. The results are reportedbelow:

He Productivity: 44,000 centiBarrer

N₂ Productivity: 4,000 centiBarrer

CO₂ Productivity: 152,000 centiBarrer

He/N₂ Selectivity: 11

CO₂ /N₂ Selectivity: 38

Example 22

To a rapidly stirred solution of 2,3,5,6-tetramethyl-1,4-phenylenediamine (32.86 g, 0.20 mol) in N-methyl pyrrolidone (500 ml) under aninert atmosphere at 50° C. was added 1,2,4,5-benzenetetracarboxylicdianhydride (8.73 g, 0.04 mol). To the resulting solution wasportionwise added 4,4'-(hexafluoroisopropylidene)-bis(phthalicanhydride) (71.04 g, 0.16 mol, in 4 portions, last portion washed inwith 250 ml N-methyl pyrrolidone). The reaction solution was allowed tostir at 50° C. overnight. A solution of acetic anhydride (75.8 ml, 0.80mol) and triethylamine (112.9 ml, 0.81 mol) was added with rapidstirring and the viscous yellow solution was allowed to stir anadditional 2 hours at 50° C. The polymer was precipitated in methanol,collected by filtration, washed with methanol,and air dried. Theoff-white polymer was further dried in a vacuum oven at room temperatureovernight, at 100° C. for 3 hours, and at 250° C. for 4 hours to give105 g product.

Example 23

Films of the polymer prepared in Example 22 were cast from a 10% polymersolution (based on weight) in N-methyl pyrrolidone onto a glass platetreated with Du Pont TEFLON® dry lubricant at 80° C. with a 15 mil(3.8×10⁻⁵) knife gap. The films were dried on the plate at 80° C. for 25minutes, cooled to room temperature, stripped off the plate and dried ina vacuum oven at room temperature overnight and at 120° C. for 4 hours.The films were then tested for mixed gas O₂ /N₂ (21/79 mole)permeabilities at 300 psig (20.7×10⁵ Pa), 25° C. The results arereported below:

O₂ Productivity: 16,400 centiBarrer

O₂ /N₂ Selectivity: 3.4 and

O₂ Productivity: 22,000 centiBarrer

O₂ /N₂ Selectivity: 3.2

Example 24

Asymmetric membranes of Example 1 were cast from a 20% (weight) polymersolution in N,N-dimethyl formamide: formamide (95:5, weight) solutiononto a glass plate at 50° C. with a 15-mil knife gap. The films weredried on the plate at 50° C. for 2 minutes and then precipitated indeionized water at 25° C. After the resulting white asymmetric membraneshad sat in deionized water for 24 hours, they were placed inisopropanol. After sitting in isopropanol for 24 hours, the membraneswere air dried. The asymmetric membranes were then tested for mixed gasO₂ /N₂ (21/79) (mole) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C.The results are reported below:

O₂ Productivity: 350 GPU

O₂ /N₂ Selectivity: 3.3 ##EQU2##

Example 25

Asymmetric membranes of Example 1 were cast from a 20% (weight) polymersolution in N,N-dimethyl formamide: water (97:3) solution (weight) ontoa glass plate at 100° C. with a 15 mil knife gap. The films were driedon the plate at 100° C. for 10 seconds and then precipitated indeionized water at 25° C. After the resulting white asymmetric membraneshad sat in deionized water for 24 hours, the membranes were placed inisopropanol for 24 hours. The membranes were air dried. The membraneswere treated with a 0.25% (weight/volume) Dow Corning SYLGARD-184®solution in FREON-113 and dried in a vacuum oven at 50° C. for 24 hours.The asymmetric membranes were then tested for mixed gas O₂ /N₂ (21/79)(mole) permeabilities at 300 psig (20.7×10⁵ Pa), 25° C. The results arereported below:

O₂ Producivity: 100 GPU

O₂ /N₂ Selectivity: 4.0

The asymmetric membranes prepared above were consecutively tested forpure gas carbon dioxide and methane permeabilities at 100 psig, 25° C.The results are shown below:

CO₂ Productivity: 1000 GPU

CH₄ Productivity: 36 GPU

CO₂ /CH₄ Selectivity: 28

I claim:
 1. A gas separation membrane formed of an aromatic polyimide ofthe formula ##STR7## where --Ar-- is ##STR8## or mixture thereof, and Ris ##STR9## or mixtures thereof and --X, --X₁, --X₂, and --X₃ areindependently alkyl groups having 1 to 6 carbon atoms or aromatic groupshaving 6 to 13 carbon atoms, and --Z is --H, --X, --X₁, --X₂, or --X₃.2. The membrane of claim 1 wherein --X, --X₁, --X₂, or --X₃ are --CH₃ or--C₂ H₅.
 3. The membrane of claim 2 wherein R is ##STR10##
 4. A processof separating two or more gases comprising of steps of bringing amixture of two or more gases into contact with one side of apermselective membrane which is formed from an aromatic polyimide of theformula ##STR11## where --Ar-- is ##STR12## or or mixtures thereof, andR' is ##STR13## or mixtures thereof and --X, --X₁, --X₂, and --X₃ areindependently alkyl groups of 1 to 6 carbon atoms, or aromatic grouphaving 6 to 13 carbon atoms and --Z is --H, --X, --X₁, --X₂, or --X₃whereby one of said gases permeates said membrane at a differentproductivity rate than at least one other gas.
 5. The process of claim 4wherein --X, --X₁, --X₂, --X₃ are --CH₃ or --C₂ H₅.
 6. The process ofclaim 5 wherein R is ##STR14##